Everything about Avgas totally explained
Avgas is a
high-octane aviation fuel used for
aircraft and
racing cars.
Avgas is a
portmanteau for
aviation gasoline, as distinguished from
mogas (motor gasoline), which is the everyday
gasoline used in
cars.
Avgas is used in aircraft that use
piston or
Wankel engines.
Gas turbines can operate on avgas, but typically do not. Turbine and
diesel engines are designed to use
kerosene-based
jet fuel.
Avgas properties and varieties
Gasoline used for aviation fuel generally has two numbers associated with its octane rating. Examples of this include the (now almost completely unavailable) 80/87 avgas, and the 100/130 avgas. The first number indicates the octane rating of the fuel tested to "aviation lean" standards, which is similar to the
Motor Octane Number (MON) rating given to automotive gasoline. The second number indicates the octane rating of the fuel tested to the "aviation rich" standard, which tries to simulate a supercharged condition with a rich mixture, elevated temperatures, and a high manifold pressure.
Avgas has a lower and more uniform
vapor pressure than mogas, which keeps it in the liquid state at high-altitude, preventing
vapor lock. The particular mixtures in use today are the same as when they were first developed in the
1950s and
1960s, and therefore the high-octane ratings are achieved by the addition of
tetra-ethyl lead (TEL), a highly toxic substance that was phased out for car use in most countries in the
1980s. The main
petroleum component used in blending avgas is
alkylate, which is essentially a mixture of various
isooctanes, and some refineries also use some
reformate.
Avgas is currently available in several grades with differing maximum lead concentrations. Since TEL is a rather expensive additive, a minimum amount of it's typically added to the fuel to bring it up to the required octane rating so actual concentrations are often lower than the maximum.
Jet fuel isn't avgas. It is similar to
kerosene and is used in turbine engines. Confusion can be caused by the terms Avtur and AvJet being used for Jet Fuel. In Europe, environmental and cost considerations have led to increasing numbers of aircraft being fitted with highly fuel-efficient diesel engines; these too run on jet fuel. Civilian aircraft use Jet-A, Jet-A1 or in severely cold climates Jet-B. There are other classification systems for military turbine and diesel fuel. See
Jet fuel.
Grades
100LL
100LL, spoken as "100 low lead", contains
tetra-ethyl lead (TEL), a
lead based
anti-knock compound, but less than the "highly-leaded" 100/130 avgas it effectively replaced. Most piston aircraft engines require 100LL and a suitable replacement fuel hasn't yet been developed for these engines. While there are similar engines that burn non-leaded fuels, aircraft are often purchased with engines that use 100LL because many airports only have 100LL. 100LL contains a maximum of 2
grams of lead per US gallon, or maximum 0.56 grams/litre and is the most commonly available and used aviation gasoline.
82UL
82UL is the specification for an unleaded fuel similar to automobile gasoline but without additives. It could potentially be used in aircraft that have a Supplemental Type Certificate for the use of automobile gasoline with an aviation lean octane rating (MON) of 82 or less or an antiknock index of 87 or less. It couldn't be used in engines that require 100LL. See
Octane Rating. The FAA highly recommends installing placards stating the use of 82UL is or isn't approved on those airplanes that specify unleaded autogas (
mogas) as an approved fuel. As of 2008 82UL isn't being produced and no refiner has announced plans to put it into production..
80/87
Prior to its phase out in the early 1990s avgas 80/87 had the lowest lead content with a maximum of 0.5
grams lead per U.S.
gallon, and was only used in low compression ratio engines.
100/130
Avgas 100/130 had a higher octane grade aviation gasoline, containing a maximum of 4 grams of lead per US gallon, maximum 1.12 grams/
litre. 100LL "low lead" has replaced avgas 100/130 in most places, but Avgas 100/130 is still sold in
Australia and
New Zealand as one of the two manufacturers in Australia is unable to make Avgas 100LL.
91/96 & 115/145
In the past other grades were also available, particularly for military use, such as avgas 115/145 and 91/96. Note that the octanes of avgas can't be directly compared to those of mogas, as a different test engine and method is used to determine the octane. The first (lower) number is the
lean mixture rating, the second (higher) number is the
rich mixture rating. For mogas, the
octane rating is typically expressed in the U.S. as an
anti-knock index (known as "pump rating"), which is the average of the octane rating based on the
research and
motor test method ((R+M)/2).
Identification dyes
Fuel dyes aid both ground crew and pilots in identifying the proper fuel grade.
80/87 was
red, 100/130 was
green, 115/145 was purple (leading to the U.S. Naval aviation slang term "grape juice" for avgas, standardized use of purple paint on aviation fueling systems shipboard in the U.S. Navy, and the distinctive purple shirts worn by flight deck refueling personnel "grapes" aboard U.S. aircraft carriers) and 100LL is
blue, while jet fuel, JET A1, is clear or straw, being undyed.
Untaxed diesel fuel for off-road use is also
dyed red.
Mogas
Gasoline (MOGAS) may be used in aircraft that have a
Supplemental Type Certificate for automotive gasoline. Most of these applicable aircraft have low-compression engines which were originally certified to run on 80/87 avgas and require only "regular" 87
anti-knock index automotive gasoline. Examples of this include the popular
Cessna 172 or
Piper Cherokee with the 150 hp variant of the
Lycoming O-320. Some aircraft engines were originally certified using a 91/96 avgas and have STC's available to run "premium" 91
anti-knock index automotive gasoline. Examples of this include some Cherokee's with the 160 hp
Lycoming O-320 or 180 hp
O-360 or the
Cessna 152 with the
O-235.
Properties
Avgas has a
density of 6.02 lb/
US gallon at 15
°C, or 0.72 kg/l, and this density is commonly used for
weight and balance computation. Density increases to 6.40 lb/US gallon at -40 °C, and decreases by about 0.5% per 5 °C increase in temperature.
Avgas has an
emission coefficient (or factor) of 18.355 pounds
CO2 per
US gallon, or about 3.05 units of weight CO
2 produced per unit weight of fuel used.
Consumption
The annual U.S. usage of avgas was 236 million gallons (893 million liters) in 2006.
Avgas compared to other fuels
Many
general aviation aircraft engines were designed to run on 80/87 octane, roughly the standard for automobiles today. Direct conversions to run on automotive fuel are fairly common and applied via the
supplemental type certificate (STC) process. However, the alloys used in aviation engine construction are rather outdated, and engine wear in the valves is a potential problem on automotive gasoline conversions. Fortunately, significant history of mogas-converted engines has shown that very few engine problems are actually caused by automotive gasoline. A larger problem stems from the wider range of allowable vapor pressures found in automotive gasoline; this can pose some risk to aviation users if fuel system design considerations are not taken into account. Automotive gasoline can vaporize in fuel lines causing a vapor lock (a bubble in the line), starving the engine of fuel. This doesn't constitute an insurmountable obstacle, but merely requires examination of the fuel system, ensuring adequate shielding from high temperatures and maintaining sufficient pressure in the fuel lines. This is the main reason why both the specific engine model as well as the aircraft in which it's installed must be supplementally certified for the conversion. A good example of this is the Piper Cherokee with high-compression 160 hp or 180 hp engines. Only later versions of the airframe with different engine cowling and exhaust arrangements are applicable for the automotive fuel STC, and even then require fuel system modifications.
Vapor lock typically occurs in fuel systems where a mechanically-driven fuel pump mounted on the engine draws fuel from a tank mounted lower than the pump. The reduced pressure in the line can cause the more volatile components in automotive gasoline to flash into vapor, forming bubbles in the fuel line and interrupting fuel flow. If an electric boost pump is mounted in the fuel tank to push fuel toward the engine, as is common practice in fuel-injected automobiles, the fuel pressure in the lines is maintained above ambient pressure, preventing bubble formation. Likewise, if the fuel tank is mounted above the engine and fuel flows primarily due to gravity, as in a Cessna high-wing airplane, vapor lock can't occur, using either aviation or automotive fuels.
In addition to vapor locking potential, automotive gasoline doesn't have the same quality tracking as aviation gasoline. To help solve this problem, the specification for an aviation fuel known as
82UL has recently been developed. This fuel would be essentially automotive gasoline that has additional quality tracking and restrictions on permissible additives. This fuel isn't currently in production and no refiners have committed to producing it.
The public comment period on this petition closed on March 17, 2008.
In a February 2008 interview Ross indicated that Continental Motors believes that the aviation industry will be "forced out" of using 100LL avgas in the near future, leaving automotive fuel and jet fuel as the alternatives.
Further Information
Get more info on 'Avgas'.
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